Method to accelerate wetting of an ion exchange membrane in a semi-fuel cell

ABSTRACT

A new treatment method for ion exchange membranes used in semi-fuel cells that accelerates the wetting of the membranes by aqueous electrolyte solutions, thus reducing the start up time for metal/hydrogen peroxide-based semi-fuel cells. Specifically, a Nafion® membrane that is intended for dry storage in a semi-fuel cell is treated with glycerin (glycerol) to enhance its rate of absorption of electrolyte solution when the semi-fuel cell is activated.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

CROSS REFERENCE TO OTHER RELATED APPLICATIONS

This patent application is co-pending with a related patent applicationentitled HIGH EFFICIENCY SEMI-FUEL CELL INCORPORATING AN ION EXCHANGEMEMBRANE (Navy Case No. 82737), by, Maria G. Medeiros, Eric G. Dow,employees of the United States government, Russell R. Bessette, Susan G.Yan, and Dwayne W. Dischert.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to semi-fuel cells, and more specificallyto a new treatment for ion exchange membranes that accelerates thewetting of the membranes by aqueous electrolyte solutions, thus reducingthe start up time for metal/hydrogen peroxide-based semi-fuel cells.

(2) Description of the Prior Art

A semi-fuel cell is essentially a hybrid of fuel cells and batteries.Combining the refillable cathode or catholyte oxidizer of fuel cellswith the consumable anode fuel of batteries. Semi-fuel cells arecurrently under investigation as electrochemical power sources forunmanned undersea vehicles. In a semi-fuel cell, a metal anode, such asmagnesium or aluminum along with a liquid catholyte, typically a strongoxidizer like hydrogen peroxide, are consumed to produce energy. Theelectrochemical reaction is given below for magnesium with hydrogenperoxide in acid media.

Anode: Mg→Mg²⁺+2e⁻ 2.37 v

Cathode: H₂0₂+2H⁺+2e⁻ 2 H₂0 1.77 v

Cell Reaction: Mg+H₂0₂+2H⁺ Mg²⁺+2 H₂0 4.14v

In addition to the primary electrochemical reaction, several parasiticreactions can also take place.

Decomposition: 2 H₂0₂→2H₂0+0₂

Direct Reaction: Mg+H₂0₂+OH⁻→Mg²⁺+3 OH⁻

Corrosion: Mg+2H₂0→Mg²⁺+2 OH⁻+H₂

Of the three parasitic reactions listed above, the direct reaction isthe most detrimental to the operation of the semi-fuel cell since bothmagnesium and hydrogen peroxide are consumed in a single step. Whereasmagnesium corrosion can be suppressed by pH adjustment and hydrogenperoxide decomposition minimized by careful temperature control,prevention of the direct reaction requires that the magnesium anode andhydrogen peroxide catholyte be physically separated from each other. Toaccomplish this, a semi-permeable membrane capable of ion exchange isplaced between the anode and cathode compartments of the semi-fuel cellin order to isolate the anolyte and catholyte solutions.

In order for ionic transport to occur across a membrane, the membranemust first absorb the electrolyte solution. The membrane's rate ofabsorption determines how quickly the semi-fuel cell reaches itsoperating voltage. In most applications, semi-fuel cells are stored“dry” to prevent corrosion of the magnesium anode. When electricalenergy is needed, the semi-fuel cell's anode and cathode compartmentsare flooded with electrolyte so that power generation can begin. Themembrane must then wet immediately upon contact with the electrolyte, sothat the semi-fuel cell can begin to supply power. In the situationwhere the semi-fuel cell is being used with an unmanned underwatervehicle, the requirement for power generation is within seconds of thevehicle's deployment. Ion exchange membranes, such as Nafion®, require along pre-soak period (at least 12 hours) in aqueous electrolyte solutionin order to be fully wet and therefore fully functional in a semi-fuelcell. A method to accelerate the wetting of the membrane is needed.

Several prior art methods exist for treating a Nafion® membrane to makeits performance more consistent and reproducible. However, thesetechniques are designed to remove impurities from the manufacturingprocess and/or exposure to the environment rather than enhance themembrane's ability to absorb liquid. Typical treatments involve boilingthe membrane in dilute acid followed by rinsing in boiling distilledwater. After treatment, the membrane must remain wet prior to use.

What is needed is a method that will allow an ion exchange membrane suchas a Nafion® membrane in a semi-fuel cell to be stored dry and then towet immediately when the semi-fuel cell is activated.

SUMMARY OF THE INVENTION

It is a general purpose and object of the present invention to provide amethod to increase the rate of absorption of electrolyte solution by aNafion® membrane.

This object is accomplished with the present invention by providing atreatment method whereby a Nafion® membrane is treated with glycerin(glycerol) to enhance its rate of absorption of electrolyte solution

BRIEF DESCRIPTION OF THE DRAWINGS

None.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The following is a detailed description of the treatment method of thepresent invention. In the preferred embodiment a membrane made ofNafion®, is used as the separating membrane in a semi-fuel cell,however, other perfluorinated ionomer membranes such as Felmion® andAciplex-XR® could also be used. In the preferred embodiment, thetreatment method is applied to Nafion® membranes NE-112, NE-1135, N-115and N-117. Initially the membrane is immersed in 1.5 M H₂S0₄ for 48hours. The membrane is then removed from the H₂S0₄ and rinsed indistilled and or deionized H₂0. The membrane is then soaked in distilledand or deionized H₂0 for 24 hours. The membrane is then removed from theH₂0 and then again rinsed in more distilled and or deionized H₂0. Themembrane is then immersed in a polyhdric alcohol for 48 hours. In thepreferred embodiment of the method the polyhdric alcohol is glycerin.After 48 hours the membrane is removed from the glycerin and once againrinsed in distilled and or deionized H₂0. Finally the membrane is driedin air for at least 48 hours prior to use.

Once treated in this manner, the Nafion® membrane may be installed inthe semi-fuel cell in the dry state and is ready to instantaneouslyabsorb electrolyte when the semi-fuel cell's anode and cathodecompartments are flooded with electrolyte, thus allowing powergeneration to occur immediately.

The advantages of the present invention over the prior art are that thepresent invention allows a Nafion® ion exchange membrane to be storeddry within a semi-fuel cell module. This is critical since the magnesiumanode of a semi-fuel cell must also be stored dry to prevent itscorrosion. Upon activation of the semi-fuel cell, the Nafion® membranetreated according to the method of the present invention instantlyallows ionic transport to readily take place between the anode andcathode compartments. The semi-fuel cell is then able to generate andmaintain sufficient voltage. In the situation where a semi-fuel cell isused to power an unmanned underwater vehicle, the present invention willreduce the vehicle's dependence on standby/auxiliary batteries.

What has thus been described is a new treatment method for ion exchangemembranes used in semi-fuel cells that accelerates the wetting of themembranes by aqueous electrolyte solutions, thus reducing the start uptime for metal/hydrogen peroxide-based semi-fuel cells.

Obviously many modifications and variations of the present invention maybecome apparent in light of the above teachings. For example differenttypes of ion exchange membranes may be used. Different polyhydricalcohols may be used.

In light of the above, it is therefore understood that within the scopeof the appended claims, the invention may be practiced otherwise than asspecifically described.

1. A method for treating a perflourinated ionomer membrane to increasesaid membrane's rate of absorption of electrolyte solution, whichcomprises: immersing said membrane in H₂SO₄; removing said membrane fromH₂SO₄; then immersing said membrane in H₂O; removing said membrane fromH₂O; and then immersing said membrane to glycerin.
 2. (canceled) 3.(canceled)
 4. A method in accordance with claim 1 wherein immersing saidmembrane in H₂SO₄ comprises immersion of the membrane in 1-2 M H₂SO₄ forat least 48 hours.
 5. A method in accordance with claim 1 whereinimmersing said membrane in H₂O comprises soaking said membrane indistilled H₂O for at least 24 hours.
 6. A method in accordance withclaim 1 wherein immersing said membrane in H₂O comprises soaking saidmembrane in deionized H₂O for at least 24 hours.
 7. A method inaccordance with claim 1 wherein immersing said membrane in glycerincomprises immersion of the membrane in glycerin for at least 48 hours.8. A method in accordance with claim 4 further comprising: rinsing saidmembrane in distilled H₂O after removing said membrane from the 1-2 MH₂SO₄ solution after the at least 48 hours have elapsed and prior toimmersing said membrane in H₂O.
 9. A method in accordance with claim 5further comprising: rinsing said membrane in distilled H₂O afterremoving said membrane from said distilled H₂O after the at least 24hours have elapsed and prior to immersing said before said membrane inglycerin.
 10. A method in accordance with claim 7 further comprising:rinsing said membrane in distilled H₂O after removing said membrane fromsaid glycerin after the at least 48 hours have elapsed.
 11. A method inaccordance with claim 10 further comprising the step of: drying saidmembrane in air for at least 48 hours after rinsing said membrane indistilled H₂O.
 12. A method for treating a perflourinated ionomermembrane to increase said membrane's rate of absorption of electrolytesolution in a semi-fuel cell, comprising the sequential steps of:immersing said membrane in 1-2 M H₂SO₄ for at least 48 hours; removingsaid membrane from the 1-2 M H₂SO₄ solution after at least 48 hours haveelapsed; rinsing said membrane in distilled and deionized H₂O; soakingsaid membrane in said distilled and deionized H₂O for at least 24 hours;removing said membrane from the distilled and deionized H₂O after atleast 24 hours have elapsed; rinsing said membrane in distilled anddeionized H₂O; immersing said membrane in glycerin for at least 48hours; removing said membrane from the glycerin after at least 48 hourshave elapsed; rinsing said membrane in distilled and deionized H₂O; anddrying said membrane in air for at least 48 hours prior to use. 13.(canceled)
 14. A method in accordance with claim 12 wherein the membraneis a perfluorinated ionomer membrane selected from the group includingNAFION®, FLEMION®, ACIPLEX-XR®.